A fingerprint sensor is an electronic device used to capture a digital image of the fingerprint pattern. There are many types of fingerprint sensors, such as optical type, ultrasonic type and capacitance type. Among all of them, the capacitance type fingerprint sensor is getting popular recently for its compact size, workability and good cost structure.
Capacitance type fingerprint sensors use principles associated with capacitance in order to form fingerprint images. They are packaged semiconductors with one side exposed to the external environment. The exposed side is used to fetch the fingerprint images. The surface of the fingerprint sensor therefore may lead electrostatics around itself and cause Electrostatic Discharge (ESD) inside and bears pressure from the fingers.
In order to settle the mentioned problems, there are many prior arts providing practical method to form protective structures for the fingerprint sensor. For ESD protection, please refer to FIG. 1. U.S. Pat. No. 6,091,082 discloses a structure and method for creating a passivation to protect an integrated circuit. The structure includes a circuit 1, a dielectric 2, and metal plates 3 over which an insulating layer 4 is disposed that electrically and hermetically isolates the circuit 1. A discharge layer 5 is deposited to form a passivation 6 that protects the circuit 1 from ESD caused by a finger. Although the structure can provide ESD protection, the passivation 6 is not rigid enough to resist scratches. Meanwhile, oil and water can easily attach to the passivation 6 to cause residual of the fingerprint.
Another improvement for ESD protection is provided in U.S. Pat. No. 6,310,371. Please refer to FIG. 2. The invention provides a fingerprint sensor chip 11 formed on a first dielectric layer 12 of a semiconductor wafer 13. The fingerprint sensor chip 11 includes a number of rectangular sensor areas 17 arranged in a matrix format which are surrounded by conductors 14, a second dielectric layer 15 covering the sensor areas 17 and the conductors 14 where the surface of the second dielectric layer 15 positioned above each of the sensor areas 17 is formed as a protruding rectangular platform 16 with a shallow trench around the platform 16, a rectangular metal plate 18 positioned on top of each of the rectangular platforms 16 which is used as a sensor plate of the fingerprint sensor chip 11, and a protective layer 19 positioned on the surface of the semiconductor wafer 13 to cover and protect the underlying circuitry. Although '371 has better ESD protection, concaveness of protective layer 19 may stick dusts. This results in poor performance of the fingerprint sensor chip 11 after it is used for a period of time.
As to resistance against pressure, U.S. Pat. No. 7,629,200 discloses a fingerprint detection device. Please refer to FIG. 3. The fingerprint detection device 20 has a fingerprint sensor chip 21 and a diamond-like carbon (DLC) film 22 covering the outermost surface of the sensor chip. The DLC film 22 provides sufficient strength and enhanced electrostatic discharge withstand voltage to the fingerprint sensor chip 21. Thus, the DLC film protects the fingerprint sensor chip 21 without any conventional protective cover. The DLC film 22 is less scratchable and less stainable. Since the fingerprint detection device 20 has no protective cover, the device can be provided in a thin and compact form. In addition, the device has high reliability. However, the DLC film 22 might not be easily attached to the fingerprint sensor chip 21. In addition, the DLC film 22 may crack due to external compact or temperature difference between itself and the fingerprint sensor chip 21.
After the introduction of prior arts for protection of fingerprint sensors is illustrated, it is necessary to provide a protective structure for fingerprint sensors with good ESD protection, excellent resistance against pressure, and stain-proof and water-proof surface. Most of all, the protective structure is free from cracking of its compositions.